The prior art discloses a magnetic read transducer referred to as a magnetoresistive (MR) sensor or head which has been shown to be capable of reading data from a magnetic surface at great linear densities. An MR sensor detects magnetic field signals through the resistance changes of a read element fabricated of a magnetic material as a function of the strength and direction of magnetic flux being sensed by the read element. These prior art MR sensors operate on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization and the direction of sense current flow through the element. A more detailed description of the AMR effect can be found in "Memory, Storage, and Related Applications", D. A. Thompson et al., IEEE Trans. Mag. MAG-11, p. 1039 (1975).
More recently, a different, more pronounced magnetoresistive effect has been described in which the change in resistance of a layered magnetic sensor is attributed to the spin-dependent transmission of the conduction electrons between the magnetic layers through a non-magnetic layer and the accompanying spin-dependent scattering of electrons at the layer interfaces and within the ferromagnetic layers. This magnetoresistive effect is variously referred to as the "giant magnetoresistive" (GMR) or "spin valve" effect. Such a magnetoresistive sensor fabricated of the appropriate materials provides improved sensitivity and greater change in resistance than observed in sensors utilizing the AMR effect. In this type of MR sensor, the in-plane resistance between a pair of ferromagnetic layers separated by a non-magnetic layer varies as the cosine (cos) of the angle between the magnetization in the two layers.
U.S. Pat. No. 4,949,039 to Grunberg describes a layered magnetic structure which yields enhanced MR effects caused by antiparallel alignment of the magnetizations in the magnetic layers. Grunberg describes the use of antiferromagnetic-type exchange coupling to obtain the antiparallel alignment in which adjacent layers of ferromagnetic materials are separated by a thin interlayer of Cr or Y.
U.S. Pat. No. 5,206,590 to Dieny et al. discloses an MR sensor in which the resistance between two uncoupled ferromagnetic layers is observed to vary as the cosine of the angle between the magnetizations of the two layers. This mechanism produces a magnetoresistance that is based on the spin valve effect and, for selected combinations of materials, is greater in magnitude than the AMR.
The U.S. Pat. No. 5,159,513 to Dieny et al. discloses an MR sensor based on the above-described effect which includes two thin film layers of ferromagnetic material separated by a thin film layer of a non-magnetic metallic material wherein at least one of the ferromagnetic layers is of cobalt or a cobalt alloy. The magnetization of the one ferromagnetic layer is maintained perpendicular to the magnetization of the other ferromagnetic layer at zero externally applied magnetic field by exchange coupling to an antiferromagnetic layer (element 18 in FIG. 2 of U.S. Pat. No. 5,159 513)
Published European Pat. Application EP-A-0 585 009 discloses a spin valve effect sensor in which an antiferromagnetic layer and an adjacent magnetically soft layer co-operate to fix or pin the magnetization of a ferromagnetic layer. The magnetically soft layer enhances the exchange coupling provided by the antiferromagnetic layer.
The spin valve structures described in the above-cited U.S. patents and European patent application require that the direction of magnetization in one of the two ferromagnetic layers be fixed or "pinned" in a selected orientation such that under non-signal conditions the direction of magnetization in the other ferromagnetic layer, the "free" layer, is oriented either perpendicular to (i.e. at 90.degree. to) or antiparallel to (i.e. at 180.degree. to) the direction of magnetization of the pinned layer. When an external magnetic signal is applied to the sensor, the direction of magnetization in the non-fixed or "free" layer rotates with respect to the direction of magnetization in the pinned layer. The output of the sensor depends on the amount of this rotation. In order to maintain the magnetization orientation in the pinned layer, a means for fixing the direction of the magnetization is required. For example, as described in the above-cited prior art documents, an additional layer of antiferromagnetic material can be formed adjacent to the pinned ferromagnetic layer to provide an exchange coupled bias field and thus pinning. Alternatively, an adjacent magnetically hard layer can be utilized to provide hard bias for the pinned layer.
Another alternative to provide a ferromagnetic layer with a fixed or pinned orientation is described in U.S. Pat. No. 5,301,079 granted to Cain et al. A magnetoresistive read sensor based on the spin valve effect is disclosed in which a sense current flowing in the sensor element generates a bias field which sets the direction of magnetization in each ferromagnetic layer at an equal, but opposite, angle .theta. with respect to the magnetic easy axis, thus providing an angular separation of 20 in the absence of an applied magnetic signal. Application to this sensor of the magnetic signal to be sensed results in a further incremental rotation of the direction of magnetization of each ferromagnetic layer, the directions of these two incremental rotations being opposite.
With regard to the above prior art, it is a principal object of the present invention to provide an MR sensor based on the spin valve effect in which neither an antiferromagnetic layer nor hard biasing, nor bias current generating circuitry is required for fixing the magnetization orientation in one or more of the ferromagnetic layers.